A Washington husband-and-wife team of political consultants, Hank and Paula Parkinson, are betting about $18,000 that thousands of fat American women will make them rich.

Mrs. Parkinson, 29, was once, by her own description, a "fat, bored, Midwest housewife." But that was 52 pounds ago, before she posed in lingerie for Playboy's recent "Women in Government" pictorial, petite and curvaceous thanks to a touch of cosmetic surgery and the help of some little red and white pills from a georgetown druggist.

This month the Parkinsons launch an advertising and direct mail campaign to market those pills under the name "Paula Parkinson's PPE-75." A $7,800 full-page ad in the mid-Atlantic issues of the November Cosmopolitan magazine features a snapshot of a chubby Paula Parkinson "at the dinner table as usual" next to a fetching picture of Mrs. Parkinson today wearing a white bikini. "From plump to Playboy," reads the ad's headline.

"Frankly, we're going to try to be as crassly commercial as possible, because here's something that really works," says Hank Parkinson, 49.

PPE-75 is an appetite suppressant called phenylpropanolamine that the owner of Georgetown's Morgans pharmacy, Harold Elwyn, suggested Mrs. Parkinson try in her crusade to reduce. The parkinsons swore by the product and cut a deal with a Florida manufacturer to market it under their name. Now they're spending about $18,000 to advertise and promote the pills.

This is a far cry from work both Parkinsons have done on behalf of political clients, such as a group of businessmen, mostly insurance agents, who are trying to block passages of the Federal Crop Insurance Act of 1979. In an interview with Karen Feld of the Independent New Alliance before the Playboy pictorial appeared, Mrs Parkinson suggested her new, slim look had complicated her lobbying efforts.

According to Feld, Mrs. Parkinson claimed more than 100 congressmen propositioned her -- a statement Mrs Parkinson denies making. Which, from a strictly marketing standpoint, is too bad; a headline suggesting PPE-75 enabled a former fatty to capture the hearts of a fifth of congress might sell as many pills as that Florida factory caould churn out.